1. Field of the Invention
The invention relates to a continuous casting mold, in particular a thin slab mold, in which the flow of liquid metal in the mold is influenced by a magnetic field which is generated by permanent magnets arranged on the mold, and wherein the permanent magnets have, over their width and/or height different magnetic strengths or are spaced from each other by different distances for a different field strength.
2. Description of the Prior Art
The use of magnetic means for braking and homogenizing the liquid metal flow is a known technique and is described in numerous technical documents. The installation components, which are described in the documents, have all large masses which make difficult the oscillation of the mold that is necessary for the operation.
The document EP O 880 417 B describes a magnetic brake for casting metal in a mold and which consists of a magnetic core and a coil supplied with permanent current or low-frequency alternating current. There is further provided a return line for closing the magnetic circuit.
The progress in the development in the field of permanent magnets (hard ferrites, rare-earth magnets) opened, meantime, new uses for possible field strengths of permanent magnets, which permanent magnets appears to be a suitable alternative for use instead of the above-described electrical magnet.
It has already been proposed to replace the electromechanical brake (EMBr) equipment, which was used up to the present for generating the magnetic field (field coils, electrical control, outer yoke for conducting the magnetic flux, etc.), with permanent magnets which are directly mounted on the mold.
The document EP 0568 579 describes a method of controlling the flow of the molten metal in a non-solidified metal region of a casting mold, wherein the mold is supplied with at least one primary flow of the molten metal and a cast strand is formed, and wherein at least one static magnetic field is generated by poles which are arranged adjacent to the mold and consist of permanent magnets. The magnetic field serves for breaking the primary flow of the molten metal flowing in the mold and for splitting the primary flow and for controlling the produced secondary flow. The magnetic field is so arranged that it acts over the entire width of the strand formed in the mold. The magnetic field should extend in a plane extending perpendicular to the cast direction and at level at which the magnetic field strength reaches its maximal value and can be varied within a range of from 60% to 100% of the maximal value, while simultaneously the field strength has a maximum value of 500 Gauss at a level with the highest outer surface/meniscus of the molten metal. The magnetic field is controlled and distributed by providing displaceable magnetic poles and/or adjustable core members.
The document EP 00 40 383 B1 describes a method of stirring the non-solidified region of a cast strand, wherein the strand is formed in a mold, and the cast steel flows through a pouring spout or directly into the mold. There, where the cast steel penetrates the melt already amassed in the mold, at least one static magnetic field is generated that brakes the cast or pouring steel and so splits it that its momentum is weakened or absorbed. The device, which is provided to this end, can be formed of one or several permanent magnets.
Document JP 08155610 discloses a rectangular mold in four corners of which permanent magnets are arranged for generating South and North magnetic fields.
Permanent magnets have a substantially smaller configuration at the same magnetic induction field strength and, therefore, a significantly reduced mass. They do not require any additional means for conducting a magnetic flux in form of an outside yoke. When necessary, it is sufficient to use ferromagnetic materials, which are available in the mold frame, for closing the magnetic flux circuit.
However, use of permanent magnets requires other special procedures. In the state of the art, permanent magnets are used as possible sources of static magnetic fields but only as equipment for the case when the magnetic field is generated by current coils with direct current DC or low-frequency alternating current, as discussed above, but not, however, for permanent magnets.
Because permanent magnets have no switch for turning on and off, they require special safety measures for installation and monitoring of the equipment. In distinction from the alternating current drive, special methods of equipment are necessary for operating a continuous casting machine.
With a magnetic brake, there are provided, on both sides of the mold opposite each other, permanent magnets for generating a magnetic field. The induction field strength at this arrangement follows, at a spacing between the permanent magnets in the intermediate space, an equation:
      B    ⁡          (      z      )        =            2      ·              B        o            ·      cosh        ⁢                  π        ·                  [                      z            -                          d              2                                ]                    h      wherein Bo is the induction field strength of one of the permanent magnets, z-distance from one of the magnets, d-distance between the magnets and h-operating height of the magnets. The operating height is determined by measurement. π is the number Pi (=3.14 . . . ), and cos is a hyperbolic cosine (see FIG. 1).
An object of the invention is to provide a continuous casting mold in which the turbulence of the mold meniscus is reduced.